Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper products have been recognized for many years by the photographic industry. The accumulation of static charge on film or paper surfaces can cause irregular static marking fog patterns in the emulsion layer. The presence of static charge also can lead to difficulties in support conveyance as well as the attraction of dust which can result in fog, desensitization, and other physical defects during emulsion coating. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) also can produce irregular fog patterns or "static marks" in the emulsion layer. The severity of static-related problems has been exacerbated greatly by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The generation of electrostatic charge during the coating process results primarily from the tendency of webs to undergo triboelectric charging during winding and unwinding operations, during conveyance through the coating machines, and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the final photographic film product. In an automatic camera, the winding of roll film out of and back into the film cassette, especially in a low relative humidity environment, can result in static charging and marking. Similarly, high-speed automated film processing equipment can produce static charging resulting in marking. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders (e.g., x-ray, graphic arts films).
It is widely known and accepted that accumulated electrostatic charge can be dissipated effectively by incorporating one or more electrically conductive "antistatic" layers into the overall film structure. Antistatic layers can be applied to one or to both sides of the film support as subbing layers either underlying or on the side opposite to the sensitized emulsion layer. Alternatively, an antistatic layer can be applied as the outermost coated layer either over the emulsion layers (i.e., as an overcoat) or on the side of the film support opposite to the emulsion layers (i.e., as a backcoat) or both. For some applications, the antistatic function can be included in the emulsion layers or pelloid layers as an intermediate layer. A wide variety of electrically conductive materials can be incorporated in antistatic layers to produce a broad range of surface conductivities. Many of the traditional antistatic layers used for photographic applications employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in Prior Art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge carrying ions are greatly reduced and the bulk conductivity is substantially decreased. At high relative humidities, an exposed antistatic backcoating can absorb water, swell, and soften. Especially in the case of roll films, this can result in a loss of adhesion between layers as well as physical transfer of portions of the backcoating to the emulsion side of the film (viz. blocking). Also, many of the inorganic salts, polymeric electrolytes, and low molecular weight surface-active agents typically used in such antistatic layers are water soluble and can be leached out during film processing, resulting in a loss of antistatic function.
One of the numerous methods proposed by prior art for increasing the electrical conductivity of the surface of photographic light-sensitive materials in order to dissipate accumulated electrostatic charge involves the incorporation of at least one of a wide variety of surfactants or coating aids in the outermost (surface) protective layer overlying the emulsion layer(s). A wide variety of ionic-type surfactants have been evaluated as antistatic agents including anionic, cationic, and betaine-based surfactants of the type described, for example, in U.S. Pat. Nos. 3,082,123; 3,201,251; 3,519,561; and 3,625,695; German Patent Nos. 1,552,408 and 1,597,472; and others. The use of nonionic surfactants having at least one polyoxyethylene group as antistatic agents has been disclosed in U.S. Pat. Nos. 4,649,102 and 4,891,307; British Patent No. 861,134; German Patent Nos. 1,422,809 and 1,422,818; and others. Further, surface protective layers containing nonionic surfactants having at least two polyoxyethylene groups have been disclosed in U.S. Pat. No. 4,510,233. In order to provide improved performance, the incorporation of an anionic surfactant having at least one polyoxyethylene group in combination with a nonionic surfactant having at least one polyoxyethylene group in the surface layer was disclosed in U.S. Pat. No. 4,649,102. A further improvement in antistatic performance by incorporating a fluorine-containing ionic surfactant having a polyoxyethylene group into a surface layer containing either a nonionic surfactant having at least one polyoxyethylene group or a combination of nonionic and anionic surfactants having at least one polyoxyethylene group was disclosed in U.S. Pat. Nos. 4,510,233 and 4,649,102. Additionally, surface or backing layers containing a combination of specific cationic and anionic surfactants having at least one polyoxyethylene group in each which form a water-soluble or dispersible complex with a hydrophilic colloid binder are disclosed in European Patent Appl. No. 650,088 and British Patent Appl. No. 2,299,680 to provide good antistatic properties both before and after processing without dye staining.
Surface layers containing either non-ionic or anionic surfactants having polyoxyethylene groups often demonstrate specificity in their antistatic performance such that good performance can be obtained against specific supports and photographic emulsion layers but poor performance results when they are used with others. Surface layers containing fluorine-containing ionic surfactants of the type described in U.S. Pat. Nos. 3,589,906; 3,666,478; 3,754,924; 3,775,236; and 3,850,642; British Patent Nos. 1,293,189; 1,259,398; 1,330,356 and 1,524,631 generally exhibit negatively charged triboelectrification when brought into contact with various materials. Such fluorine-containing ionic surfactants exhibit variability in triboelectric charging properties after extended storage, especially after storage at high relative humidity. However, it is possible to reduce triboelectric charging from contact with specific materials by incorporating into a surface layer other surfactants which exhibit positively charged triboelectrification against these specific materials. The dependence of the triboelectrification properties of a surface layer on those specific materials with which it is brought into contact can be somewhat reduced by adding a large amount of fluorine-containing nonionic surfactants of the type disclosed in U.S. Pat. No. 4,175,969. However, the use of a large amount of said fluorine-containing surfactants results in decreased emulsion sensitivity, increased tendency for blocking, and increased dye staining during processing. Thus, it is extremely difficult to minimize the level of triboelectric charging against all those materials with which an imaging element may come to contact without seriously degrading other requisite performance characteristics of the imaging element.
The inclusion in a surface or backing layer of a combination of three kinds of surfactants, comprising at least one fluorine-containing nonionic surfactant, and at least one fluorine-containing ionic surfactant, and a fluorine-free nonionic surfactant has been disclosed in U.S. Pat. No. 4,891,307 to reduce triboelectric charging, prevent dye staining on processing, maintain antistatic properties on storage, and preserve sensitometric properties of the photosensitive emulsion layer. The level of triboelectric charging of surface or backing layers containing said combination of surfactants against dissimilar materials (e.g., rubber and nylon) is alleged to be such that little or no static marking of the sensitized emulsion occurs. The incorporation of another antistatic agent such as colloidal metal oxide particles of the type described in U.S. Pat. Nos. 3,062,700 and 3,245,833 into the surface layer containing said combination of surfactants was also disclosed in U.S. Pat. No. 4,891,307.
The use of a hardened gelatin-containing conductive surface layer containing a soluble antistatic agent (e.g., Tergitol 15-S-7), an aliphatic sulfonatetype surfactant (e.g., Hostapur SAS-93), a matting agent (e.g., silica, titania, zinc oxide, polymeric beads), and a friction-reducing agent (e.g., Slip-Ayd SL-530) for graphic arts and medical x-ray films has been taught in U.S. Pat. No. 5,368,894. Further, a method for producing such a multilayered photographic element in which the conductive surface layer is applied in tandem with the underlying sensitized emulsion layer(s) is also claimed in U.S. Pat. No. 5,368,894. A surface protective layer comprising a composite matting agent consisting of a polymeric core particle surrounded by a layer of colloidal metal oxide particles and optionally, conductive metal oxide particles and a nonionic, anionic or cationic surfactant has been disclosed in U.S. Pat. No. 5,288,598.
Antistatic layers incorporating electronic rather than ionic conductors also have been described extensively in the prior art. Because the electrical conductivity of such layers depends primarily on electronic mobilities rather than on ionic mobilities, the observed conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Antistatic layers containing conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films or networks are well known in the prior art. Of the various types of electronic conductors previously described, electroconductive metal-containing particles, such as semiconductive metal oxide particles, are particularly effective. Fine particles of crystalline metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies are sufficiently conductive when dispersed with polymeric film-forming binders to be used to prepare optically transparent, humidity insensitive, antistatic layers useful for a wide variety of imaging applications, as disclosed in U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276,; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,368,995; 5,382,494; 5,459,021; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, zinc antimonate, indium antimonate, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Of these, the semiconductive metal oxide most widely used in conductive layers for imaging elements is a crystalline antimony-doped tin oxide, especially with a preferred antimony dopant level between 0.1 and 10 atom percent Sb (for Sb.sub.x Sn.sub.1-x O.sub.2) as disclosed in U.S. Pat. No. 4,394,441.
An electroconductive protective overcoat overlying a sensitized silver halide emulsion layer of a black-and white photographic element comprising at least two layers both containing granular conductive metal oxide particles and gelatin but at different metal oxide particle-to-gelatin weight ratios has been taught in Japanese Kokai A-63-063035. The outermost layer of said protective layer contains a substantially lower total dry coverage of conductive metal oxide (e.g., 0.75 g/m.sup.2 vs 2.5 g/m.sup.2 ) present at a lower metal oxide particle-to-gel weight ratio (e.g., 2:1 vs 4:1) than that of the innermost conductive layer.
The use of electroconductive antimony-doped tin oxide granular particles in combination with at least one fluorine-containing surfactant in a surface, overcoat or backing layer has been disclosed broadly in U.S. Pat. Nos. 4,495,276; 4,999,276; 5,122,445; 5,238,801; 5,254,448; and 5,378,577 and also in Kokai Nos. A-07-020,610 and B-91-024,656. The fluorine-containing surfactant is preferably located in the same layer as the electroconductive tin oxide particles to provide improved antistatic performance. A surface protective layer or a backing layer comprising at least one fluorine-containing surfactant, at least one nonionic surfactant having at least one polyoxyethylene group, and optionally one or both of electroconductive metal oxide granular particles or a conductive polymer or conductive latex is disclosed in U.S. Pat. No. 5,582,959. The addition of said electroconductive metal oxide particles to a subbing, backing, intermediate or anti-halation layer was disclosed in a particularly preferred embodiment. Further, the addition of a nonionic surfactant having at least one polyoxyethylene and a fluorine-containing surfactant each either singly or in combination to a surface protective layer or a backing layer was disclosed in another particularly preferred embodiment. However, the inclusion of electroconductive metal oxide particles in a surface protective layer was neither taught by examples nor claimed.
Similarly, a silver halide photographic material comprising an outermost layer overlying a sensitized silver halide emulsion layer containing an organopolysiloxane and a nonionic surfactant having at least one polyoxyethylene group, optionally combined with or replaced by one or more fluorine-containing surfactants or polymers, and a backing layer containing electroconductive metal oxide particles is disclosed in U.S. Pat. No. 5,137,802. The backing layer is located on the opposite side of the support from said outermost layer overlying the emulsion layer. The incorporation of an organopolysilane, a nonionic surfactant having a polyoxyethylene group and/or a fluorine-containing surfactant or polymer in said outermost layer was disclosed as providing excellent antistatic performance with a minimum degree of deterioration with storage time, and negligible occurrence of static marking.
As indicated herein above, the prior art for electrically-conductive overcoat layers containing ionic surfactants or combinations of ionic and nonionic surfactants and for antistatic layers containing electrically-conductive metal oxide particles useful for imaging elements discloses a wide variety of overcoat layer compositions. However, there is still a critical need in the art for a conductive overcoat which not only effectively dissipates accumulated electrostatic charge, but also minimizes triboelectric charging against a wide variety of materials with which the imaging element may come into contact. In addition to providing superior antistatic performance, the conductive overcoat layer also must be highly transparent, must resist the effects of humidity change, strongly adhere to the underlying layer, exhibit suitable mushiness, not exhibit ferrotyping or blocking, not exhibit adverse sensitometric effects, not impede the rate of development, not exhibit dusting, and still be manufacturable at a reasonable cost. It is toward the objective of providing such improved electrically-conductive, non-charging overcoat layers that more effectively meet the diverse needs of imaging elements, especially of silver halide photographic films, than those of the prior art that the present invention is directed.